Fuel cell start optimization

Abstract

A method of starting a fuel cell stack in subzero conditions that minimizes start times while avoiding cell reversal by using an iterative model to determine the optimal current density time profile for startup.

1. A method of managing a startup of a fuel cell stack to limit a minimum cell voltage and optimize a startup time, the method comprising the steps of:
 a. measuring a stack temperature in response to a request for stack startup; 
 b. specifying a current density time profile based upon the stack temperature, a stored lowest membrane hydration and a stored average membrane hydration, wherein current density varies as time raised to the power of n (time″); 
 c. calculating a voltage and a power density performance of an average cell with the average membrane hydration and a lowest cell with the lowest membrane hydration; 
 d. if the average cell voltage is less than a predetermined minimum average voltage or if the lowest cell voltage is less than a predetermined minimum low voltage, reducing a ramp rate and repeating said steps b. and c.; 
 f. if the average cell voltage is greater than the minimum average voltage plus a predetermined amount and the lowest cell voltage is greater than the minimum low voltage, increasing the ramp rate and repeating said steps b. and c.; and 
 g. if the average cell voltage is in a range of the minimum average voltage plus the predetermined amount and the lowest cell voltage is equal to or greater than the minimum low voltage, using the current density time profile to start the stack. 
 
     
     
       2. The method according to  claim 1  including steps of purging the stack, measuring one of voltage and high frequency resistance response of each cell of the stack and determining a membrane hydration of each cell based on the measured one of voltage and high frequency resistance response. 
     
     
       3. The method according to  claim 1  including a step of identifying and storing the determined lowest membrane hydration and average membrane hydration. 
     
     
       4. A method of optimizing a startup time of a fuel cell stack comprising the steps of:
 a. purging a fuel cell stack during stack shutdown; 
 b. measuring one of voltage and high frequency resistance response of each cell of the stack after purging; 
 c. determining membrane hydration of each cell based upon the measurements performed in said step b.; 
 d. identifying a lowest cell with a lowest membrane hydration; 
 e. determining an average membrane hydration of the stack; 
 f. measuring a stack temperature in response to a request for stack startup; 
 g. specifying a current density time profile based upon the stack temperature, the lowest membrane hydration and the average membrane hydration, wherein current density varies as time raised to the power of n (time″); 
 h. calculating a voltage and a power density performance of an average cell with the average membrane hydration and the lowest cell with the lowest membrane hydration; and 
 i. repeating said steps g. and h. by changing a ramp rate until the average cell voltage is in a range of a minimum average voltage plus a predetermined amount and the lowest cell voltage is equal to or greater than a minimum low voltage and then using the current density time profile to start the stack. 
 
     
     
       5. The method according to  claim 4  including if the average cell voltage is less than a predetermined minimum average voltage or if the lowest cell voltage is less than a predetermined minimum low voltage, performing said step i. by reducing the ramp rate. 
     
     
       6. The method according to  claim 4  including if the average cell voltage is greater than the minimum average voltage plus a predetermined amount and the lowest cell voltage is greater than the minimum low voltage, performing said step i. by increasing the ramp rate. 
     
     
       7. A method of managing the startup of a fuel cell stack to limit the minimum cell voltage and optimize the startup time, the method comprising the steps of
 a. purging a fuel cell stack during stack shutdown; 
 b. measuring one of voltage and high frequency resistance response of each cell of the stack after purging; 
 c. determining membrane hydration of each cell based upon the measurements performed in said step b.; 
 d. identifying cell with lowest membrane hydration; 
 e. determining an average membrane hydration of the stack; 
 f. measuring a stack temperature in response to a request for stack startup; 
 g. specifying a current density time profile based upon the stack temperature, the lowest membrane hydration and the average membrane hydration, wherein current density varies as time raised to the power of n (time″); 
 h. calculating a voltage and a power density performance of an average cell with the average membrane hydration and a lowest cell with the lowest membrane hydration; 
 i. if the average cell voltage is less than a predetermined minimum average voltage or if the lowest cell voltage is less than a predetermined minimum low voltage, reducing a ramp rate and repeating said steps g. and h.; 
 j. if the average cell voltage is greater than the minimum average voltage plus a predetermined amount and the lowest cell voltage is greater than the minimum low voltage, increasing the ramp rate and repeating said steps g. and h.; and 
 k. if the average cell voltage is in a range of the minimum average voltage plus the predetermined amount and the lowest cell voltage is equal to or greater than the minimum low voltage, using the current density time profile to start the stack.